Cementation of a casing in an oil well consists of placing a sheath of cement in the annular space between the convex side of the casing and the wall of the hole. The hole may be formed with another casing or with the rock. This cement sheath has an essential role in the stability and isolation of oil wells.
The cement sheath is obtained by pumping a cement slurry made from cement, water and adjuvants. This cement slurry is in the liquid state when it is pumped. Hydration of the cement particles leads the liquid slurry towards a solid state, characterized by the existence of a backbone and pores, thereby forming a porous medium.
The cement sheath is exposed to various mechanical and thermal stresses, also called bottom conditions, during the lifetime of the well, from operations conducted in the well (pressure tests, mud changing, cold and hot stimulations, production of reserves . . . ) or from phenomena directly arising in the subsoil (compaction of the reservoir, earthquakes . . . ) and this until it is abandoned, or even beyond this. These stresses may damage the constitutive material of the cement sheath, degrade its mechanical and hydraulic properties and therefore modify its contribution to the stability and seal of the well.
Knowledge of the behavior of the cement under bottom conditions and of the time-dependent change of this behavior is essential for analyzing the operation of the well during its drilling, its exploitation and for guaranteeing its seal for storing and sequestering gas (CH4, C2H6, CO2, for example) in underground reservoirs. More generally, it is necessary to be able to conduct mechanical or physical tests on materials obtained by curing compositions (and notably cement compositions) under very specific conditions which are those encountered in the wells, i.e. generally absence of air and high pressure. These materials are actually very different from those obtained by curing compositions of the same type under ambient conditions (i.e. in air and under atmospheric pressure).
Many techniques have been proposed for characterizing the mechanical behavior of such materials. A first category of techniques covers static mechanical tests on samples which are cured in ageing benches at a given pressure and temperature, which are then unloaded in order to position them in a measurement apparatus. The unloading step requires bringing back the samples to atmospheric pressure and to room temperature, which may not only damage the samples but also perturb the determination of the characteristics of said samples.
A second category of techniques covers dynamic tests based on indirect measurement of wave propagation and not comprising any return of the samples under ambient conditions. These techniques however have a limited benefit because of their indirect nature: in particular the static parameters have to be obtained from dynamic parameters by using correlation formulae; these formulae are themselves obtained by static tests, which may be marred with errors, or even not cover the field of application of the tested materials. A third category of techniques comprises a few proposals of static mechanical tests without the detrimental unloading and reloading step mentioned above.
Thus, document EP 1541987 describes a system in which a cement composition is cast into a bone-shaped mold, the sample is aged under temperature and under pressure and uniaxial tensile loading is carried out until breakage of the sample, without having to unload the sample. However, with this method, it is not possible to carry out the measurements under bottom conditions, the pressure can only be exerted on both faces of the sample and the other faces are subject to a loading condition by reaction of the mold and not by application of stress under bottom conditions. The measurements are therefore biased. Further, only tensile tests are possible, but the latter are biased as regards the measurement of the elastic constants relatively to the compression tests, because of the occurrence of microcracks which invalidate the elasticity assumption. The range for determining the elastic parameters is therefore highly reduced. Further, it is not possible to measure the breakage parameters in compression and finally the geometry used is unconventional.
Document U.S. Pat. No. 7,621,186 describes an alternative of the previous system, adopting a geometry of the frusto-conical type. It therefore suffers from the same drawbacks. Document WO 2007/020435 proposes a technique which consists of having the cement composition set in an annular space located between two concentric tubes, and of then varying the pressures on the concave side of the inner tube and/or on the convex side of the outer tube while measuring the induced deformations. This technique only allows confinement compression tests (radial direction), and not axial compression tests. Further, this technique has the drawback of being based on a measurement in a heterogeneous field of stresses (in elasticity, the stress and deformation fields in a hollow cylinder vary as 1/r2). Thus the measurement of the elastic properties of the sample are highly inaccurate (very sensitive to errors), just like that of the damaging and breakage properties of the sample.
Document U.S. Pat. No. 7,089,816 describes a technique which consists of having the cement composition set in a cylindrical casing (consisting of a deformable membrane and of two pistons) placed in a confinement enclosure, and then of directly proceeding with the mechanical tests by applying the confinement pressure through the membrane and axial loading via the pistons, just like for a conventional triaxial cell. But in reality, the use of a flexible membrane for the setting of the cement does not give the possibility of obtaining a sample with a regular shape after setting. Because of the changes in volume associated with setting, there actually occur Taylor instabilities leading to the sample losing its initial geometry. Also, with this technique it is not possible to carry out measurements in line with the existing procedures, hydration of the cement is not properly reproduced.
Document U.S. Pat. No. 7,549,320 corresponds to a technique of the same type, with a change in the loading technology. In particular, the document further proposes to have the sample set in a flexible membrane. The rigid compartment surrounding the flexible membrane is intended for applying fluids but it does not have an influence on the shape of the sample during the setting.
Document U.S. Pat. No. 7,552,648 further describes another alternative, wherein fluid is injected into the sample itself, which is porous, in order to obtain the desired pressure. A tensile test is then carried out. No compression test is provided, and the provision of an exterior fluid does not properly simulate the hydric exchanges under bottom conditions.
Therefore, there exists a need for having a novel technique for testing cement samples (or other curing compositions) not having the above drawbacks. In particular, there exists a need for having a technique available with which measurements of mechanical, hydraulic or physico-chemical properties may be carried out under bottom conditions during the curing or even beyond this, without again passing through atmospheric temperature and pressure conditions, while controlling the shape of the sample, and without being limited to an unconventional geometry.